The angel food cakes symbolize the neighbors' compassion and meddling.

Bob Ewell attacked Scout and Jem Finch when they were walking home, and Boo Radley killed him.  Realizing this, Sheriff Heck Tate and Atticus decide to tell everyone that Bob Ewell fell on his knife.  They do not want to tell anyone that Boo Radley was the one who saved the children, because Boo is such a private person that he would not want anyone to know.

Heck Tate explains why.

“… All the ladies in Maycomb includin‘ my wife’d be knocking on his door bringing angel food cakes. To my way of thinkin’, Mr. Finch, taking the one man who’s done you and this town a great service an‘ draggin’ him with his shy ways into the limelight—to me, that’s a sin. It’s a sin and I’m not about to have it on my head. If it was any other man, it’d be different. But not this man, Mr. Finch.” (Ch. 30)

Boo Radley would hate it if the people of Maycomb knew what he had done.  They would be curious.  They would talk.  They would want to be neighborly, and come have a look at the hero.  Boo Radley would prefer to stay in the shadows.  He saved Scout and Jem because he cared about them.  Atticus and Sherrif Tate saved him from having to face an endless supply of neighborly attention.

The angel food cake is a symbol of neighborly compassion or neighborly meddling.  People bring it to show that they care, but it also puts them in contact with the person who is the object of the gossip.  This is why Heck Tate does not want to tell anyone what Boo Radley did.  Boo does not want any angel food cake, and he does not want any nosy neighbors.  He is an extremely private person.